This is a proposal to continue study of two recently discovered, heterochromatin-specific proteins. The first protein D1, has recently been identified by us as a specific component of nucleosomes containing (A+T)-rich D. melanogaster satellite DNA [2-4]. This is the first example of a heterochromatin-specific nucleosomal protein. The second protein (tentatively called an Alpha-protein) has recently been detected and partially purified by us from african green monkey (CV-1) cells as a protein that preferentially binds the green monkey Alpha-satellite DNA (a tandem repeat of 172 bp). The two precedents above make it likely that many if not most of tandemly repetitive ("satellite") DNA families have specific DNA-binding proteins associated with the corresponding regions of chromatin. Since such proteins are likely to be responsible for at least some of the unique and still poorly understood properties of tandemly repetitive heterochromatin, we propose studying the D1 and Alpha proteins in detail, and using them as probes to address the heterochromatin problem. The proposed studies include detailed chemical and physico-chemical characterization of D1 and Alpha proteins; preparation and use of D1- and Alpha-specific immunological reagents; cloning or genes for D1 and Alpha proteins, and analysis of specific interactions of D1 and Alpha proteins with DNA, nuclear proteins and nucleosomes. Proposed systematic studies on D1 and Alpha proteins in Drosophila and green monkey, respectively, should provide a new way to address the problem of heterochromatin organization; such studies may also illuminate more general principles of chromosome organization and function.